Sift the flour, baking powder and salt together in a small bowl and set aside.In a large bowl, combine the butter and sugar. Using a hand mixer set to medium, blend until smooth. Add the egg yolks and egg and blend well. Add the vanilla and blend. In three stages, add the dry ingredients to the wet, mixing well after each addition. Scrape down the sides of the bowl as you go.Divide the dough in half- it will be crazy sticky- and press each half between two large sheets of parchment paper or cling film. Roll each dough half out into 8”-wide circles, then place the circles on a plate or baking sheet and freeze for 1 hour, or chill in the fridge for at least 3.
While the dough is chilling, make the pastry custard. Combine the milk and heavy cream over medium heat. While this is heating, whisk together the egg yolks, sugar and cornstarch until pale yellow. Slowly pour the hot milk into this mixture, whisking constantly. Pour and scrape everything back into the saucepan and turn the heat to low. Continue whisking as it cooks and begins to thicken. If any lumps appear, whisk them away. When the custard is very thick and the consistency of jam, remove it from the heat and stir in the vanilla. Pour the custard into a bowl and press a layer of cling film on top. Refrigerate for ½ an hour.
Once the dough is chilled, preheat the oven to 350*F. Heavily grease an 8” springform pan and lock the sides into place. Lay one dough half on the bottom of the pan, pressing the dough against the sides of the pan in an even layer.
Pour the custard onto the middle of the dough and spread out, using a spatula or a cake spreader. It will be cold, but will warm up as you spread. Once the pastry custard is in an even layer, add the cherries, spreading them as evenly as you can.
Lay the second dough half on top and press the sides down. Use the pastry spreader to tuck the dough together but don’t stress too much about this; the dough will fuse while baking.
If you’d like, draw a crosshatch on the top of the dough with a sharp knife or the tines of a fork. Brush the crust with egg white wash or water and bake for 40-45 minutes, until golden brown.
Remove from the oven and loosen the sides of the springform pan. Allow to cool at least 10 minutes before serving.
Cake is best if eaten the day it’s made. It will keep for about a day if tightly wrapped and chilled.